Styrene-butadiene-styrene and styrene-isoprene-styrene blends have been routinely used as a base for elastic film formulations used in personal care applications. It is noted that blends of SBS and SIS exhibit tensile strength properties of >1600 psi while maintaining good hysteresis at both 100% and 300%, with recovered energy of greater than 70% and a permanent set less than 10% for 100% hysteresis. After 300% hysteresis, a recovered energy of greater than 60% and a permanent set less than 20% is embodied by the present invention.
It is known that SBS block copolymers increase in viscosity during melt processing. Likewise, it is known that SIS block copolymers decrease in viscosity during melt processing. With the state of the art formulations comprising a blend of SBS and SIS, undesirable gel formation generally results, due to degradation of the SBS copolymers, caused by cross linking of the butadiene. Gel formation produces unsightly and inconsistent film defects. Isoprene degradation can also occur and when it does chain scission results and the isoprene fragments thereby reduce the viscosity. Thus with the state of the art composition it is always a balance to have just the right amount of SBS and SIS such that the gel formation is balanced against the fragmentation and the viscosity remains more consistent. This balance is extremely difficult to achieve because the rates of degradation of SBS and SIS are not the same.
It is known through discovery by the present inventor that random S-I/B-S neat copolymers have excellent viscosity stability because the butadiene does not degrade as easily as the butadiene in SBS copolymer. However, copolymers of S-I/B-S have low tensile strength.
What is sought in the marketplace, in the art of elastic film and fiber formulations, is an SBC which has sufficient tensile properties in addition to good viscosity stability. The present invention completely satisfies these desires of the marketplace.